Field of the Invention
The present invention relates to an ophthalmic lens, for a pair of spectacles, comprising an antifouling coating and a temporary top-coat deposited beforehand on the antifouling coating, the temporary top-coat making the lens fit for edging and leading, after it has been removed, to an ophthalmic lens having improved antifouling properties relative to ophthalmic lenses obtained from ophthalmic lenses comprising a conventional temporary top-coat.
Description of the Related Art
A lens results from a succession of molding and/or surfacing/polishing operations, which define the geometry of the two (convex and concave) surfaces of said lens, and then appropriate surface treatments.
The edging operation, which consists in machining the edge face or periphery of a lens so as to give this lens the dimensions required to fit it in the spectacle frame into which it is intended to be placed, is the last of the finishing steps carried out on the ophthalmic lens.
The edging is generally carried out with a mill comprising diamond wheels and/or cutting heads that perform the machining such as defined above.
The lens is held, during this operation, by axially acting blocking members.
The relative movement of the lens, with respect to the wheel, is controlled, generally digitally, in order to produce the desired shape.
As will be apparent, it is absolutely essential for the lens to be firmly held during this movement.
To do this, before the edging operation, the lens is blocked, i.e. a holding means or block is positioned on the convex surface of the lens.
A blocking pad, possibly a self-adhesive pad, for example comprising a double-sided pressure-sensitive adhesive (PSA), is placed between the block and the convex and/or concave surface (preferably the convex surface) of the lens—the latter case will be described in more detail in the remainder of the present application.
The lens thus equipped is positioned on one of the aforementioned axial blocking members, the second axial blocking member then chucking the lens on its concave face via a stop that is generally made of an elastomer. During machining, a tangential torque is applied to the lens, this possibly causing the lens to rotate with respect to the block if the system for holding the lens.
Whether the lens is held correctly mainly depends on whether there is a good bond at the interface between the holding pad and the convex surface of the lens.
Antifouling coatings, also referred to as “hydrophobic and/or oleophobic coatings” are well known in the art. They are generally manufactured from fluorosilanes or fluorosilazanes, i.e. silanes or silazanes containing fluorine atoms.
The highest performance antifouling coatings possess low surface energies, typically surface energies lower than or equal to 14 mJ/m2 and preferably lower than or equal to 12 mJ/m2.
The consequence of these low surface energies is that the lens may slip off axis during the edging step.
This technical problem is known.
In order to remedy it, it has been proposed to deposit temporary coatings (or temporary top-coats) that allow slip during the edging to be prevented while preserving the properties of the antifouling coating.
Such techniques have been the subject matter of many patents such as patents EP 1 392 613, WO 2004/110946 and WO 2008/053020.
After the top-coat has been removed, the properties of the hydrophobic layer may, even if they are well preserved on the whole, sometimes be worse than those of the layer before deposition of the temporary layer. In particular, contact angle may be smaller by a few degrees, relative to that of the hydrophobic layer before deposition of the temporary layer.
Moreover, durability (how long the coating keeps its hydrophobic properties under the effect of repeated wiping) may be affected.